The present invention relates to an apparatus for reproducing signals recorded on a magnetic recording medium and, more particularly, to a magnetic playback apparatus arranged to detect during signal reproduction a signal magnetic field from the magnetic recording medium in the form of a change in high-frequency characteristics of a magnetic material.
A magnetic playback system is known which uses a magnetoresistance effect according to which a variation in electromagnetic characteristics of a magnetic material due to a magnetic field from a magnetic recording medium generated by a recorded signal is detected to reproduce the recorded signal.
Another playback system suitable for high-density recording and playback is disclosed in Japanese Patent Early Publication No. 57-36407. This system uses a variation of a tensor permeability .mu.' and its loss term .mu." as a change in high frequency characteristics of a magnetic material, which is caused by recorded magnetic field. A magnetic material, such as ferrite, permalloy, or amorphous alloy, is used for a playback head. A high frequency circuit is coupled with the playback head. To reproduce the recorded signal, an output voltage of the high frequency circuit, which varies due to a change of the circuit impedance, is detected and rectified.
Generally in a high frequency magnetic field, .mu.' and .mu." of a magnetic material are changed due to low magnetic field loss observed in the nonsaturation region of the magnetic material, and the ferromagnetic resonance absorption occuring in the saturation region of the magnetic material in a high magnetic field. The change of .mu.' and .mu." due to the ferromagnetic resonance is much greater than that due to the low magnetic field loss. Therefore, if the ferromagnetic resonance is employed, a slight change of the magnetic field can greatly change the impedance of the high frequency circuit, thus permitting a playback at a very high sensitivity and a good S/N ratio. The above publication describes the playback system using the ferromagnetic resonance absorption.
For actually sensing a recorded magnetic field by using the ferromagnetic resonance, an operation point must be set to cause a great variation of .mu.' and .mu." resulting from the ferromagnetic resonance. To realize this, a great bias magnetic field is needed in addition to the recorded magnetic field. Providing a magnetic source adjacent to the magnetic recording medium will possibly create some problems such as demagnetization of the recorded magnetic field depending on the intensity and the direction of the bias magnetic field.